In U.S. Pat. No. 5,576,624 I disclosed a metal detector apparatus which is capable of substantially reducing signals arising from ground mineralisation containing magnetic soils. Several models utilizing these principles have been commercialised and used highly successfully by gold prospectors and for land mine detection. Whilst the improvement compared to other extant metal detectors was considerable, further improvements relating to improved signal-to-noise ratio and bipolar transmit fields are disclosed herein.
The current de-mining metal detectors products utilizing the principles of my patent are unacceptable to some military organizations because of their uni-polar transmitted field; these organisations require metal detectors to transmit net zero magnetic fields so that, so called xe2x80x9csmart minesxe2x80x9d, which respond to slow changes in the environmental static magnetic field (for example caused by perturbations to the earth""s field by moving tanks), are not detonated by the metal detector. Gold prospectors always wish to find gold at greater depths.
An object of this invention is to improve upon the ability of metal detectors to locate targets at greater depths, and to produce a bi-polar magnetic field of zero net flux.
In U.S. Pat. No. 6,5766,241 disclosed a metal detector apparatus which transmits a pulse induction waveform.
The advantages of pulse sequences containing pulses of different duration are disclosed. The received signal response from magnetic soils is approximately the same for all such soils (except for the overall amplitude).
In contrast, the response from metal targets vary substantially. Hence if a proportion of one part of the received signal is subtracted from a linear combination of a different part of the received signal, it is possible to select the coefficients of the said linear combination such that the response from magnetic soils are cancelled. In general, a particular said linear combination resulting in magnetic soil signal cancellation, a first linear combination, will not result in cancellation of most metal targets, except for some particular metal targets with a particular eddy current time constant decay, a first eddy current decay time constant, which happens to also result in a zero first linear combination result. In order to detect targets with the said first eddy current decay time constant, a different linear combination of different sampled receive signals, a second linear combination, is required to cancel magnetic soil response and not cancel the said first eddy current time constant decay signal. Likewise the said second linear combination will cancel a particular metal eddy current decay signal, a second eddy current time constant decay, which will not be cancelled by the said first linear combination if chosen appropriately. Thus by simultaneously measuring and calculating at least two different linear combinations, all metal targets may be detected and magnetic soil signals approximately cancelled.
In U.S. Pat. No. 5,576,624 I disclosed a metal detector apparatus which is insensitive to induced e.m.f. signals from movement of a search coil relative to the earth""s magnetic field and magneto fields from magnetic soils and rocks. This is achieved by selecting the total integrated receive signal over a complete cycle to be zero.
An object of this invention is to improve the efficiency of a metal detector apparatus of this general type.
In one form of this invention it can be said to reside in a metal detector apparatus including a transmit coil adapted to transmit a magnetic field when operating with electrical current flowing through the said transmit coil, transmit timing control circuitry, and receive signal circuitry and receive signal processing circuitry;
and at least two power supplies, a first and a second power supply,
one adapted to provide a low voltage when operating of between 3 and 30V,
and the other adapted to provide a high voltage when operating of between 75 and 400V,
a first switch being adapted to connect the coil to the said first power supply, which may be via a diode, or diodes or passive components or both,
a second switch being adapted to connect the coil to the said second power supply, which may be via a diode, or diodes or passive components or both,
a third diode or switch being adapted to connect the said transmit coil to the said second power supply, which may De via other diodes or passive components when a back e.m.f. from the coil causes current flow in one particular direction,
a fourth diode or switch being adapted to connect the said transmit coil to the said second power supply, which may be via other diodes or passive components when a back e.m.f. from the coil causes current flow in another particular direction,
the timing sequence adapted to be generated within the transmit timing control circuitry being adapted to produce control signals for the said switches,
and the said timing sequence being adapted and otherwise connected so as to control the said switches such that there are periods when no current flows through the transmit coil.
In preference, in a further form of this invention, this can be said to reside in a metal detector apparatus including a transmit coil adapted to transmit a magnetic field when operating with electrical current flowing through the said transmit coil, transmit timing control circuitry, and at least two power supplies, a first and a second power supply, wherein the voltage of the one supply is different to the other when operating, and receive signal circuitry and receive signal processing circuitry;
and at least two switches,
a first switch being adapted to connect the coil to the said first power supply, which may be via a diode, or diodes or passive components or both,
a second switch being adapted to connect the call to the said second power supply, which may be via a diode, or diodes or passive components or both,
a third diode or switch is adapted to connect the said transmit coil to a third voltage supply, when current resulting from a back e.m.f. from the coil is flowing,
the timing sequence generated within the transmit timing control circuitry is adapted to produce control signals for the said switches, wherein there are at least two different transmit periods, a shot and a long period,
and the said timing sequence being adapted to control the said switches such that there are periods when no current flows through the transmit coil,
and the said timing sequence Is selected to control the said switches such that an average transmit coil energy at termination of the long periods is similar to that of an average transmit coil energy at termination of the short periods,
and the said timing sequence being further arranged to control the said switches such that the long pulse is at least 3 times the duration of the short pulse.
In preference one power supply is replaced by a direct connection to ground potential and the timing sequence generated within the transmit timing control circuitry is adapted to produce control signals for the switches, wherein there are at
least two different transmit periods, a short and a long period;
wherein during the long period, the first switch is pulsed on and off, and the second switch is turned on at least during periods when the first switch Is turned off.
In preference, in a further form of this invention it can be said to reside in a metal detector apparatus including a transmit coil adapted to transmit a magnetic field when operating with electrical current flowing through the said transmit coil, transmit timing control circuitry, and at least one power supply, a fourth power supply, and receive signal circuitry and receive signal processing circuitry:
and at least two switches;
a fourth switch, is adapted to connect the coil to the said third power supply, which may be via a diode, or diodes or passive components or both,
and a fifth switch, is adapted to connect the coil to 0 volts or ground potential, which may be via a diode, or diodes or passive components or both,
a sixth diode or switch Is adapted to clamp the said transmit coil voltage to a fourth voltage supply, when current resulting from a back e.m.f. from the coil is flowing,
and the timing sequence generated within the transmit timing control circuitry is adapted to produce control signals for the said switches, wherein there are at least two different transmit periods, a short and a long period,
wherein during the long period, the fourth is pulsed on and off and the fifth switch is turned on at least during periods when the fourth switch is tuned off,
and the said timing sequence is selected to control the said switches such that there are periods when no current flows through the transmit coil,
and the said timing sequence is selected to control the said switches such that the total transmit coil energy at the termination of the long periods is similar to that of the total transmit coil energy at the termination of the short periods,
and the said timing sequence is selected to control the said switches such that the long pulse is at least 3 times the duration of the short pulse.
In preference, in a further form of this invention it can be said to reside in a metal detector apparatus Including a transmit coil adapted to transmit a magneto field when operating with electrical current flowing through the said transmit coil, transmit timing control circuitry, and receive signal circuitry and receive signal processing circuitry;
and at least two power supplies, a fifth and a sixth power supply,
wherein the voltage of the one supply is different to the other when operating,
a sixth switch is adapted to connect the coil to the said fifth power supply, which may be via a diode, or diodes or passive components or both,
a seventh switch is adapted to connect the coil to the said sixth power supply, which may be via a diode, or diodes or passive components or both,
an eighth diode or switch is adapted to clamp the said transmit coil voltage to the said sixth power supply, when current resulting from a back e.m.f from the coil is flowing in one particular direction,
a ninth diode or switch is adapted to clamp the said transmit coil voltage to the fifth power supply, when current resulting from a back e.m.f. from the coil is flowing in another particular direction,
the timing sequence generated within the transmit timing control circuitry is adapted to produce control signals for the said switches, wherein there are at least two different transmit periods, a short and a long period,
and the said timing sequence is adapted to control the said switches such that there are periods when no current flows through the transmit coil,
and the said timing sequence is selected to control the said switches such that the total transmit coil energy at the termination of the long periods Is similar to that of the total transmit coil energy at the termination of the short periods,
and the said timing sequence is selected to control the said switches such that the long pulse is at least 3 times the duration of the short pulse,
and the e.m.f. of the fifth power supply is at least double that of the sixth power supply.
In preference one power supply is replaced by a direct connection to ground potential the timing sequence generated within the transmit timing control circuitry is adapted to produce control signals for the said switches, wherein there are at least two different transmit periods, a short and a long period;
wherein during the long period, the sixth is pulsed on and off, and the seventh switch is turned on at least during periods when the sixth switch is turned off.